The present invention relates generally to mode switch control systems for hybrid transmissions operable in a plurality of transmission modes, and more particularly to a mode switch control system for a hybrid transmission operable in a high speed mode for infinitely variable high speed ratios and in a low speed mode for infinitely variable low speed ratios, which is configured to smoothly switch the transmission mode from the high speed mode to the low speed mode so as not to cause an undue change in the transmission output torque and a discontinuity in a change in the transmission gear ratio.
In recent years, there have been disclosed various hybrid transmissions. One such hybrid transmission includes a differential unit interconnecting an engine, an output shaft, and motor/generators. The differential unit includes a plurality of rotating elements, three or more of which are each configured to be connected to an associated one of the engine, the output shaft, and the motor/generators. The hybrid transmission includes torque-transmitting mechanisms such as a clutch and a brake, which are selectively engageable to connect two associated ones of the rotating elements, to establish a high speed mode suitable for infinitely variable high speed ratios, or to establish a low speed mode suitable for infinitely variable low speed ratios. A Published Japanese Patent Application No. 2000-62483 shows such a hybrid transmission with a high speed mode and a low speed mode as discussed above. However, there has been disclosed no substantial appropriate process of the mode switch operation between the high speed mode and the low speed mode of the hybrid transmission. Accordingly, it is a general method of switching the transmission mode of a hybrid transmission to instantaneously switch engagement states of associated selectively engageable torque-transmitting mechanisms between an engaged condition and a disengaged condition, at a time point when the operating state of the hybrid transmission is in an intermediate one between the high speed mode and the low speed mode.